THE   UNIVERSITY 

OF  ILLINOIS 

LIBRARY 


AGRICULTURAL 
LIBRARY 


BULLETIN  No.  220 


BLACK  SPOT  OF  ONION  SETS 


BY  FEANK  LINCOLN  STEVENS 
AND  ESTHER  YOUNG  TRUE 


TIRBANA,  ILLINOIS,  MAY,  19]  9 


CONTENTS  OF  BULLETIN  No.  220 

PAGE 

INTRODUCTION 507 

THE  DISEASE 507 

The  Three  Types 508 

THE  CAUSAL  FUNGUS 509 

Culture  Studies 509 

The  Subicle 510 

The  Tubercle 510 

THE  PERITHECIUM    511 

CORRELATION  WITH  EARLIER  MORPHOLOGICAL  WORK 529 

TAXONOMIC  POSITION  OF  THE  ASCIGEROUS  STAGE 529 

SUGGESTIONS  REGARDING  CONTROL „ 531 

BIBLIOGRAPHY  .  .   532 


BLACK  SPOT  OF  ONION  SETS 

VOLUTELLA  CIRCINANS  AND  ITS  PERITHECIAL  FORM, 
CLEISTOTHECOPSIS  CIRCINANS 

BY  FRANK  LINCOLN  STEVENS,  PROFESSOR  OF  PLANT  PATHOLOGY,  UNIVER- 
SITY OF  ILLINOIS,  AND  ESTHER  YOUNG  TRUE,  RESEARCH  ASSISTANT, 
IN  COOPERATION  WITH  THE  DEPARTMENT  OF  HORTICULTURE 

INTRODUCTION 

Serious  losses  of  onions  during  storage,  particularly  of  onion 
sets,  led  to  the  present  study.  Loss  in  storage,  especially  of  the 
white  varieties,  is  very  high.  It  is  estimated  that  in  the  Chicago 
district  alone,  for  the  1917  season,  there  was  a  loss  of  approximately 
150,000  bushels  of  sets  valued  at  about  $300,000. 

In  the  present  study  onions  and  onion  sets  were  examined  cither 
directly  in  the  markets  and  stores  or  by  sample  lots  which  were  sup- 
plied by  large  holders.  Results  of  such  examinations  showed  the 
presence  of  several  fungi  apparently  as  the  cause  of  loss  in  storage, 
as  follows:  "Vermicularia,"  Sclerotium,  Botrytis  (several  species),, 
Fusarium,  Aspergillus,  Bacillus,  and  a  few  apparently  saprophytic 
forms.  The  so-called  "  Vermicularia"  was  present  in  60  to  80  per- 
cent of  the  white  onion  sets  examined,  and  Sclerotium  in  2  to  7 
percent.  The  other  fungi  appeared  in  a  lesser  quantity.  It  is  to  the 
fungus  found  most  generally  present  that  chief  attention  has  been 
given  in  the  studies  reported  in  this  bulletin. 

THE  DISEASE 

This  disease  is  easily  recognized  by  both  macroscopic  and  mi- 
croscopic observation.  As  a  disease  it  was  first  described  under  the 
name  "  V ermicularia,  circinans"  by  Berkeley  in  Gardener's  Chronicle 
in  1851. ia  It  has  later  been  referred  to  by  Clinton,2  Cooke,3  Hal- 
sled,4  Massee,5  Selby,0  Stevens,7  Stevens  and  Hall,8  Stoneman,9 
Thaxter,10  Tubeuf  and  Smith,11  Voglino,12  and  a  few  others.  It  has 
been  reported  under  many  common  names,  such  as  anthracnose,  rot, 
smudge,  surface  rot  of  the  bulb,  mold,  vermiculariose,  black  spot, 
and  neck  rot.  The  records  of  the  Plant  Disease  Survey,  U.  S.  De- 
partment of  Agriculture,  show  its  distribution  in  the  following 
states :  Connecticut,  Indiana,  Louisiana,  Maryland,  Massachusetts, 
Minnesota,  New  York,  Ohio,  Pennsylvania,  Texas,  and  Wisconsin. 

"Superior  figures  are  used  thruout  this  bulletin  to  indicate  the  literature  cita- 
tions given  in  the  bibliography,  page  532. 

507 


508  BULLETIN    No.    220  [May, 

THE  THREE  TYPES 

The  disease  assumes  three  distinctly  different  aspects,  which  are 
here  described  separately  as  Types  A,  B,  and  C. 

Type  A. — This  type  of  the  disease  is  the  most  common  and  the 
most  striking  in  appearance.  It  is  the  one  that  is  usually  regarded 
as  typical  and  the  one  commonly  described  and  figured  as  the  onion 
disease  due  to  "  Vermicularia. "  Upon  the  dry  outer  scale  of  the 
bulb  it  presents  an  approximately  circular  spot,  usually  about  the 
size  of  a  dime  or  a  quarter.  The  hue  is  prevailingly  black,  owing  to 
much  black  mycelium  within  the  tissues,  and  numerous  black  knots 
of  mycelium  are  seen,  often  grouped  in  a  series  of  concentric  circles, 
as  shown  in  Fig.  1  (1).  (See  also  Massee,5  Fig.  130,  and  Thaxter,10 
Fig.  44.) 

Variations  from  this  typical  concentric  arrangement  are  met. 
In  some  instances  the  prevailing  direction  of  the  mycelium  is  at 
right  angles  to  the  main  veins,  resulting  in  transverse  black  bands 
(see  cover  illustration).  In  other  instances  neither  the  concentric 
nor  the  banded  'arrangement  predominates  but  the  spots  consist  of 
irregularly  arranged  minute  specks,  as  illustrated  in  Fig.  1(2).  In 
still  other  instances  the  area  involved  in  the  spot  is  entirely  black 
and  the  minute  component  black  specks  are  lost  to  view,  as  in  Fig.  1 
(3,  4).  (See  also  Clinton,2  Plate  20,  .Fig.  a).  Another  view  of  the 
condition  shown  in  Fig.  1  (1)  and  in  the  cover  illustration  is 
shown  in  Fig.  2  (1).  In  many  of  these  spots  there  is  a  pronounced 
tendency  for  the  fungus  to  follow  the  course  of  the  veins,  as  may  be 
seen  in  Fig.  2  (2).  All  of  the  varying  forms  of  Type  A  occur  on 
the  outermost  scales  of  the  bulb,  as  in  Fig.  2  (3),  and  in  all  cases 
on  scales  which  are  perfectly  dry. 

Type  B. — This  type,  like  Type  A,  occurs  on  the  dry  outer  scales 
but  it  is  usually  limited  to  the  upper  part  of  the  bulb,  as  illustrated 
in  Fig.  3(1);  at  least  it  always  starts  at  the  top  of  the  bulb  and 
progresses  toward  the  base.  This  type  may  assume  any  of  the  vari- 
ations mentioned  under  Type  A.  Several  views  are  shown  in  Figs.  4 
and  5.  The  essentially  distinguishing  character  of  this  type  is  that 
the  invasion  of  the  fungus  starts  at  the  top  of  the  bulb,  usually  stop- 
ping with  the  drying  of  the  scale  before  the  equatorial  region  is 
reached.  It  is  probable  that  Type  B  results  from  infection  thru  the 
wounds  caused  in  removing  the  "top." 

Type  C. — This  type  is  manifest  in  those  scales  of  the  bulb  which 
are  still  succulent  and  juicy  or  which  remained  so  for  considerable 
time  after  the  invasion  by  the  fungus.  As  shown  in  Fig.  3  (2),  the 
spots  may  lie  directly  under  and  originate  from  spots  of  Type  A, 
but  on  account  of  their  very  different  appearance  and  character 
this  type  is  described  separately.  Fig.  3  (3)  shows  Type  A;  Fig.  3 


1919]  BLACK  SPOT  OP  ONION  SETS  509 

(4)  shows  the  same  bulb  as  Fig.  3  (3)  but  with  the  outer  scale  re- 
moved, showing  Type  C  below.  In  Type  C  the  fungus,  owing  to  the 
fact  that  it  lives  and  develops  for  a  longer  time  within  the  juicy  tis- 
sue, attains  developmental  stages  not  shown  in  the  more  limited  life 
period  in  the  outer  scales.  The  chief  manifestation  of  this  develop- 
ment is  in  the  formation  of  tubercles,  or  warty  growths.  These  are 
usually  located  at  the  centers  of  the  infected  areas,  whether  the  spot 
is  small,  2  to  3  mm.,  or  larger,  .5  to  1  cm.  In  the|  older,  larger  spots, 
secondary  or  tertiary  series  of  tubercles  may  occur,  often  in  roughly 
concentric  arrangement.  The  tubercles  are  frequently  1  or  even 
2  mm.  in  diameter  and  rise  fully  1  mm.  above  the  surrounding  scale. 
The  distinguishing  character  of  this  type  is  the  presence  of  these 
tubercles  and  the  fact  that  the  disease  often  remains  progressive, 
tho  of  course  scales  with  the  tubercular  development  may  later  dry 
and  thus  inhibit  further  progress. 

That  these  three  types  of  disease  are  due  to  the  same  fungus  is 
fully  borne  out  by  the  study  of  the  culture  characters,  by  the  micro- 
scopic studies  detailed  below,  and  by  the  fact  that  the  types  to  an 
extent  merge  one  into  the  other. 

THE  CAUSAL  FUNGUS 

The  mycelium  is  rather  coarse,  3.6  to  10.8  ^  wide,  branches  fre- 
quently and  irregularly,  and  is  cut  by  septa  at  irregular  intervals, 
usually  about  12  to  16  ^  apart.  It  is  crooked,  intercellular,  and 
intracellular,  or  even  superficial  (Figs.  8,  9,  10).  When  young  it 
is  hyaline,  later  turning  quite  dark.  The  fungus  shows  a  strong 
tendency  to  develop  mats  of  parallel  hyphae,  as  illustrated  in  Fig.  7 
(3).  It  is  these  that  are  largely  responsible  for  the  banded  appear- 
ance, shown  in  the  figure  on  the  front  cover.  It  is  also  characteris- 
tic of  the  mycelium  to  produce  branches,  the  terminal  cells  of  which 
turn  dark  and  swell,  producing  an  effect  much  like  that  of  the  so- 
called  appressoria  of  the  germinating  anthracnose  spores.  Inter- 
calary cells  also  often  darken  and  swell,  as  do  frequently  whole 
series  of  cells. 

CULTURE  STUDIES 

Studies  of  the  fungus  in  agar  showed  general  agreement  with 
the  facts  stated  above.  The  methods  used  in  isolating  the  fungus 
were:  direct  planting  on  plates  of  small  pieces  of  infected  onion 
tissue;  dilution  plating  of  spores  in  agar;  and  direct  planting  of 
spores  which  had  previously  germinated.  For  most  of  the  studies  an 
onion-broth  agar  was  used. 

When  white  onions  were  used  in  the  agar,  the  growth  of  fungus 
was  abundant ;  when  yellow  and  red  onions  were  used,  the  growth 
was  slight  and  not  many  sporing  bodies  developed.  In  Petri-dish 


510  BULLETIN    No.    220  [May, 

cultures  the  fungus  produced  a  light-colored  mycelium  near  the  up- 
per surface  of  the  medium  and  under  some  conditions  an  aerial 
mycelium.  Contrasting  the  structure  with  that  found  on  the  onion 
in  nature,  the  cells  of  the  mycelium  were  much  more  elongated  and 
very  seldom  was  there  an  appearance  of  oil  drops  in  them.  More- 
over, on  agar  there  were  very  few  of  the  dark,  swollen  end  cells  so 
characteristic  of  the  fungus  under  natural  conditions.  As  the  col- 
ony became  older,  the  aerial  mycelium  gradually  shriveled  and  the 
colony  assumed  a  gray  or  grayish-brown  color  with  numerous  small 
protuberances  on  the  surface.  These  are  the  sporodochia,  acervulus- 
like  structures  on  subicles  of  interwoven  hyphae.  They  were  not 
confined  to  the  upper  surface  alone  but  often  developed  below  the 
surface  of  the  agar.  Many  of  the  sporodochia  were  seen  to  have  a 
pedicel,  or  neck,  arising  from  the  subicle.  As  the  fungus  grew  older, 
it  often  colored  the  medium  a  dark  brown.  A  portion  of  an  agar 
plate  showing  a  colony  is  shown  in  Fig.  15,  The  small  black  dots 
seen  are  the  sporodochia. 

THE  SUBICLE 

As  the  fungus  ages,  the  mycelium  in  numerous  points  develops 
subicles,  which  are  at  first  one  cell  thick.  Various  stages  are  shown 
in  Figs.  6  (1,  2,  3),  7  (3),  and  12  (1).  The  subicle  may  attain 
dimensions  of  224  to  784  p  by  336  to  1,344  /x.  Often  setae  appear 
upon  it,  varying  in  number  from  one  to  many,  as  shown  in  Figs.  6 
(1,  3)  and  7  (3).  Fig.  12  (1)  shows  one  of  these  setigerous  subicles 
in  section.  The  setae  are  125  to  240  fi  long,  about  4  p.  wide  at  the 
base,  acute  at  the  apex,  and  very  dark  in  color. 

THE  TUBERCLE 

"When  the  fungus  grows  for  a  sufficient  time  in  the  still  juicy 
scale,  numerous  host  cells  as  well  as  intercellular  spaces  become 
filled  and  largely  distended  with  mycelium,  as  shown  in  Fig.  8 
(1,  2,  3).  This  results  in  greatly  increasing  the  thickness  of  the  host 
at  these  points,  producing  distinctly  swollen  spots  (Figs.  9  and  10). 
The  subicles,  present  superficially  over  these  regions,  increase  in 
thickness  by  division  of  the  cells  in  a  direction  tangential  to  the  sur- 
face of  the  onion,  resulting  in  distinct  rounded  protuberances,  or 
tubercles.  Several  stages  in  the  development  of  these  structures  arc 
shown  in  Fig.  11  (1,  2,  3,  4). 

The  Sporodocliium. — This  is  seen  to  consist  of  a  pseudoparenchy- 
matous  inner  tissue  covered  by  a  continuous  surface  layer.  It  may 
or  may  not  bear  setae  in  the  early  stages  of  development.  The  young 
sporodochium  eventually  ruptures  its  covering  membrane.  Occa- 
sionally the  surface  is  overgrown  by  loose  mycelial  masses,  as  shown 
in  Fig.  12  (2). 


19 19]  BLACK  SPOT  OF  ONION  SETS  511 

When  sufficient  growth  time  allows,  especially  when  one  of  the 
deeper  scales  of  the  onion  is  affected,  the  sporodochium  increases 
largely  in  diameter,  reaching  even  more  than  2  mm.,  and  becomes 
nearly  flat-topped  or  corrugated,  as  shown  in  Figs.  13  (2),  14,  and  16. 
In  all  cases  the  conidiophores  are  borne  upon  a  raised  superficial 
base  which  constitutes  the  sporodochium  (see  Figs.  13  (1,  2),  14,  and 
16),  in  contradistinction  to  the  innate  form  of  the  acervulus,  which 
has  no  such  base.  The  tubercular  swelling,  due  to  the  massing  of 
mycelium  below  and  in  the  epidermis,  partakes  of  sporodochial  char- 
acter also,  and  while  this  subepidermal  part  may  not  be  regarded  as 
constituting  a  true  sporodochium,  it  serves  to  emphasize  the  tendency 
of  the  fungus  to  produce  such  structures. 

Conidiophores. — Soon  after  rupturing  the  covering  membrane, 
the  sporodochium  develops  on  its  surface  numerous  parallel  coni- 
diophores of  palisade  arrangement,  as  shown  in  Figs.  12  (3)  and  13 
(1,  2,  3).  Interspersed  with  the  conidiophores  are  few  or  many 
setae.  The  conidiophores  are  24  to  48  /x  long,  2.4  p.  thick,  straight, 
simple,  hyaline,  few-septate,  obtuse,  and  bear  the  conidia  acro- 
genously. 

Conidia. — The  conidia  are  falcate,  acute  at  each  end,  continuous, 
hyaline,  19.2  to  26.4  /x  by  3.6  to  7.2  /x.  In  germination  they  become 
one-septate  and  send  out  germ  tubes  from  one  or  both  cells,  from 
either  the  side  or  the  tip  of  the  spore.  Appressoria  similar  to  those 
produced  by  Gloeosporium  are  also  found  in  abundance. 

THE    PERITHECIUM 

Onion  sets  which  were  heavily  infected  with  the  fungus  under 
discussion  were  placed  in  sand  in  the  greenhouse.  Upon  examination 
of  certain  of  these  sets  after  about  thirty  days,  black  bodies  were 
seen  on  the  outer  scales.  Some  of  these  were  imbedded  for  micro- 
tome sectioning;  others  were  examined  direct. 

These  structures,  the  perithecia,  are  irregularly  globular,  dark 
brown  to  black,  and  from  89.6  to  313.6  p.  in  extreme  dimensions. 
The  surface  is  characteristically  reticulated,  as  seen  in  Fig.  17  (1), 
and  there  are  often  numerous  short  hairs  growing  from  the  surface 
cells.  The  perithecia  are  borne  either  in  or  on  the  host  tissue.  Or- 
ganically connected  with  the  perithecia  is  the  typical  mycelium  of 
the  Volutella,  a  point  that  is  especially  clear  in  the  study  of  young 
perithecia.  Fig.  17  (3).  There  is  no  ostiole.  Microtome  sections 
show  the  perithecia  in  early  stages  to  resemble  sclerotia  in  general 
structure,  i.  e.,  the  whole  structure  is  pseudoparenchymatous,  the  cells 
of  the  outer  layer  being  darker  and  thicker-walled  than  those  of  the. 
interior.  This  point  is  illustrated  in  Fig.  17  (1  and  4).  In  a  later 
stage  an  ascogeiums  region  is  differentiated.  This  assumes  the 


512  BULLETIN    No.   220 

form  of  numerous  filiform  hyphae  of  nearly  parallel  direction  aris- 
ing from  the  basal  portion  of  the  perithecium,  as  shown  in  Fig.  18  (2) . 
Perithecia  of  this  age  when  crushed  release  this  ascogonium  en  masse. 
In  still  older  perithecia  the  central  part  is  filled  with  asci,  lying  in 
close  contact  and  without  paraphyses,  as  seen  in  Fig.  18  (3).  In 
still  later  stages  the  asci  disappear  and  the  ascospores  lie  free  in  the 
perithecial  cavity.  Figs.  18  (4)  and  19.  In  Fig.  17  (4)  is  shown 
a  perithecium  in  a  very  early  stage  of  development ;  general  views 
of  ascigerous  perithecia  are  shown  in  Figs.  18  (3,  4)  and  19.  The 
asci  are  here  seen  to  arise  approximately  from  a  region  which  may 
be  designated  as  basal,  i.  e.,  not  in  Plectascinaceous  fashion.  The 
asci  are  about  72  to  96  p,  by  19.2  to  24  /x  and  are  evanescent,  disappear- 
ing early. 

The  spores  are  dark,  obtuse  at  each  end,  24  to  36  /x  by  9.6  to 
14.4  p.,  muriform,  usually  with  4  to  7  /*  transverse  and  1  to  2  /x  longi- 
tudinal septa,  as  seen  in  the  optical  section  shown  in  Fig.  17  (2). 
Two  variant  spores  were  found  (in  situ  in  microtome  section),  each 
with  one  septum,  Fig.  17  (2).  These  would  be  regarded  as  immature 
except  for  the  fact  that  they  were  of  mature  color.  It  is  probable 
that  the  spores  do  really  show  this  extreme  variation  in  septation 
and  that  the  septa  may  increase  greatly  in  number  even  after  ma- 
turity of  the  spores. 

The  evidence  that  these  perithecia  belong  to  the  Volutella  may 
be  summarized  as  follows:  (1)  they  occurred  on  sets  badly  infected 
with  the  Volutella;  (2)  no  other  fungi  or  other  types  of  mycelium 
were  seen  to  be  connected  with  them;  (3)  when  studied  in  various 
stages  of  development,  the  typical  Volutella  mycelium,  which  offers 
definite  characters  for  recognition,  was  seen  in  organic  connection 
with  them,  as  illustrated  in  Fig.  18  (1)  ;  (4)  the  outgrowths  from 
the  perithecia  are  like  those  of  the  Volutella. 


FIG.  1. — VOLUTELLA,  TYPE  A:  (1)  SHOWING  CONCENTRIC  ARRANGEMENT  OF 
FUNGUS  KNOTS;  (2)  BLACK  SPECKS  IRREGULARLY  ARRANGED  THRU  THE  SPOTS; 
(3)  DIFFUSE,  NEARLY  STRUCTURELESS,  BLACK  BLOTCHES;  (4)  SAME  AS  (3) 


513 


FlG.     2. VOLUTELLA,     TYPE     A:       (1)     OTHER     VIEWS     OF     PORTIONS     SHOWN 

IN   FlG.   1    (1)    AND  IN   THE   COVER  ILLUSTRATION;      (2)    ILLUSTRATING  A    TENDENCY 

OP  THE  FUNGUS  TO  FOLLOW  A  VEIN;  (3)  A  PORTION  OF  THE  DRY  OUTER  SCALE 
HAS  BEEN  REMOVED  SHOWING  THE  FLESHY  SCALE  BELOW  To  BE  CLEAN  AND 
HEALTHY 


514 


FIG.  3. — VOLUTELLA:  (1)  TYPE  B.  THE  SPOT  is  LIMITED  TO  THE  TOP  OF 
THE  BULB;  (2)  TYPE  C.  SPOT  IN  A  LIVING  SCALE  BELOW  A  SPOT  SUCH  AS  SHOWN 
IN  FIG.  1  (1);  (3)  SHOWING  CHARACTERISTIC  TYPE  A,  ALSO  TYPE  C  BENEATH; 
(4)  TYPE  C.  SAME  AS  (3)  BUT  WITH  THE  OUTER  SCALE  REMOVED 


515 


FlG.    4. VOLUTELLA,    TYPE    B.      SHOWING    VARIATIONS    IN    THE    TYPE 


FIG.  5. — VOLUTELLA,  CHARACTERISTIC  TYPE  B 
516 


FIG.  6. —  (1)  A  SUBICLE  BEARING  SEVERAL  SETAE,  H.P.  ;  (2)  SUBICLE  SIMILAK 
BUT  WITHOUT  SETAE,  H.P.  ;  (3)  SUBICLE  IN  VERY  YOUNG  STAGE  BEARING  ONE 
SETA,  H.P. 


517 


FIG.  7. —  (1)  PHOTOGRAPH  OF  TUBERCLES  WITH  SETAE  AS  SEEN  IN  SIDE  VIEW; 
(2)  A  VIEW  OP  SETOSE  TUBERCLE  FROM  ABOVE;  (3)  SHOWING  MATS  OF  PARALLEL 
HYPHAE  DEVELOPED  IN  EARLY  STAGES,  ALSO  SUBICLES  OF  COMPARATIVELY  LARGE 
SIZE,  SOME  WITH  SETAE 


518 


FIG.  8. —  (1)  SUPERFICIAL  MYCELIUM  ON  A  DRIED  SCALE,  H.P.  ;  (2,  3)  INTERNAL 
MYCELIUM  IN  A  DRIED  SCALE,  H.P. 


519 


FIG.  9. — SHOWING  A  SUPERFICIAL  SUBICLE  OVER  A  VEIN,  H.P. 


FIG.  10. — DRIED  SCALE,  VOLUTELLA,  TYPE  A.  A  SECTION  SHOWING  THAT  THE 
FUNGUS  Is  EPIDERMAL  AS  WELL  AS  SUBEPIDERMAL  AND  SUBCUTICULAR.  SHOWING 
ALSO  THE  EELATION  OF  THE  FUNGUS  TO  THE  VEIN,  H.P. 


520 


FIG.  11. —  (1,  2,  3,  4)    EARLY  STAGES  IN  THE  DEVELOPMENT  OF 
THE  TUBERCLE,  H.P. 

521 


FIG.  12. — (1)  A  SETIGEROUS  SUBICLE  IN  SECTION,  H.P.  ;  (2)  VIEW  TO  SHOW 
THE  THICKENING  OF  THE  ONION  SCALE  BY  THE  FUNGUS,  THE  DEPOSITION  OF  MASSES 
OF  FUNGUS  MYCELIUM,  AND  THE  RELATION  TO  THE  SPORODOCHIUM!,  L.P.  ;  (3)  A 
SMALL  SPORODOCHIUM  WITH  SETAE  AND  CONIDIOPHORES  ON  A  DRIED  ONION 
SCALE,  H.P. 


522 


FIG.  13. — A  TUBERCLE  WHICH  HAS  EUPTURED  AT  THE  TOP  PREPARATORY  TO 
BECOMING  CONIDIIFEROUS  AND  SETIGEROUS,  H.P.  ;  (2,  3)  MEDIAN  SECTION  THRU  A 
SETIGEROUS,  CONIDIIFEROUS  SPORODOCHIUM.  A  DETAIL  OF  SETAE,  CONIDIOPHORES, 

AND  CONIDIA,  H.P. 

523 


y 


• 


FIG.   14. — A  LARGE  SETIGEROUS,  CONIDIIFEROUS  SPORODOCHIUM  IN  MEDIAN 
SECTION,  L.P.  •" 


FIG.  15. — PHOTOGRAPH  OF  A  PORTION  OF  AN  AGAR  COLONY   X10 


524 


FIG.  16. — MEDIAN  SECTION  THRU  AN  OLD,  LARGE  SPORODOCHIUM.  A  LOW- 
POWER  DRAWING  SHOWING  SHAPE  AND  KELATION  TO  THE  SUBSTRATUM;  THE  POR- 
TIONS A  AND  B  ENLARGED  TO  SHOW  CELLULAR  STRUCTURE,  H.P. 


525 


FIG.  17. —  (1)  A  PERITHECIUM  IN  TANGENTIAL  SECTION  SHOWING  SURFACE 
CELLS,  ALSO  CELLS  JUST  BELOW  THE  SURFACE,  H.P.;  (2)  ASCOSPORES,  H.P.;  (3)  A 
VERY  YOUNG  PERITHECIUM,  H.P. ;  (4)  A  VERY  YOUNG  STAGE  IN  THE  DEVELOPMENT 
OF  A  PERITHECIUM,  MEDIAN  SECTION  SHOWING  ATTACHMENT  TO  A  BASAL  STRUC- 
TURE, H.P. 


526 


527 


FIG.  19. — MATURE  PERITHECIUM  IN  MEDIAN  SECTION.    SPORES  FREE,  H.P. 


528 


1919}  BLACK  SPOT  OF  ONION  SETS  529 

CORRELATION   WITH  EARLIER  MORPHOLOGICAL  WORK 

Thaxter  in  1889  gave  the  first  American  account  of  this  fungus, 
placing  it  in  the  genus  "Vermicularia"  because  he  believed  it  to  be 
identical  with  a  fungus  described  as  "Vermicidaria  circinans"  by 
Berkeley  in  Gardener's  Chronicle,  in  1851,  tho  Thaxter  clearly  was 
not  satisfied  that  the  fungus  was  really  of  pycnidial  nature.  The 
structure  which  Thaxter  figures  as  "  perithecium "  in  Fig.  4510  is 
clearly  identical  in  character  with  that  shown  in  the  accompanying 
Fig.  7(1),  and  from  neither  Thaxter 's  figure,  this  figure,  nor  the  slides 
made  in  these  studies  is  there  any  reason  for  regarding  it  as  a  pycni- 
dium.  There  is  at  no  time  in  its  growth  a  covered  conidial  cavity,  nor 
in  fact  a  conidial  cavity  of  any  kind.  The  structure  is  a  tubercle  with 
a  differentiated  cortical  outer  layer.  This  outer  layer  ruptures  and 
the  tubercle  develops  as  a  sporodochium. 

*» 

These  facts  exclude  the  fungus  from  the  Sph'aeropsidales  and 
from  Vermicularia  and  place  it  in  the  Tuberculariaceae  under 
Volutella. 

Bertha  Stoneman,9  in  1898,  made  the  following  statement  re- 
garding this  fungus: 

' '  There  is  not,  however,  a  perithecium  developed  and  altho  the  fungus  has 
been  placed  among  the  Sphaeropsideae,  the  character  of  the  pustule  shows  a  close 
resemblance  to  those  species  of  Colletotrichum  in  which  an  abundant  basal  stroma 
is  developed,  while  the  marginal  setae  and  the  elevated  basidia,  as  well  as  the 
characters  in  artificial  cultures,  intimately  associate  the  fungus  with  the  genus 
Volutella. ' ' 

The  authors  fully  concur  in  this  view  so  far  as  the  conidial 
stage  is  concerned. 

That  the  fungus  under  artificial  culture  may  develop  acervuli 
rather  than  sporodochia  has  led  Voglino,12  followed  by  Walker,"  to 
class  it  as  Colletotrichum.  In  the  minds  of  the  present  writers  this 
course  is  not  justified,  since  under  its  natural  conditions  the  fungus 
produces  a  well-marked  sporodochium. 

TAXONOMIC   POSITION   OF  THE  ASCIGEROUS   STAGE 

The  classification  of  the  ascomycete  presents  certain  difficulties. 
It  has  the  general  appearance  of  a  Pleospora  so  far  as  the  character 
of  the  ascus  and  the  fully  developed  mature  spores  go,  but  the  peri- 
thecium is  distinctly  unlike  a  Pleospora  in  that  it  has  no  ostiole,  it 
is  usually  hairy,  and  in  general  structure  it  is  like  an  ascigerous  cav- 
ity developed  by  a  sclerotium  rather  than  the  usual  perithecium. 

"Walker,  J.  C.  Abstract  of  paper  presented  at  the  annual  meeting  of  the 
American  Phytopathological  Society,  New  York  City,  Dec.,  1916,  Phytopath. 
Vol.  7,  No.  1.  1917. 


530  BULLETIN    No.    220  [May, 

The  variation  in  spore  septation  is  also  noteworthy  and  does  not 
accord  well  with  a  Pleospora. 

In  general,  the  perithecium  and  the  mycelium  agree  well  with 
the  Perisporiaceae,  and  in  that  family  with  the  genus  Cleistotheca,  a 
genus  with  only  one  representative  and  that  recorded  as  a  sapro- 
phyte. Comparison  of  the  material  obtained  in  these  studies  with 
the  excellent  figure  of  Zukal13  gives  many  points  in  common.  The 
differences  are  chiefly  in  spore  septation,  which  is  irregular  in  the 
present  species ;  in  the  arrangement  of  the  asci ;  and  in  the  conidial 
stages,  Cleistotheca  possessing  a  Stachybotryos  while  this  species  has 
a  Volutella  as  the  conidial  stage.  Were  it  not  for  the  difference  in 
conidial  forms  the  authors  would  place  their  fungus  in  the  genus 
Cleistotheca.  However,  since  the  conidial  forms  are  so  different,  there 
is  proposed  for  the  perithecial  fungus  a  new  genus: 

Cleistothecopsis  gen.  nov. 

Characters  Uke  Cleistotheca  except  that  its  eonidial  stage  is  a 
Volutella.  Type  Cleistothecopsis  circinans. 

Perithecia  superficial,  irregularly  globular,  dark  brown  to 
black,  no  ostiole,  surface  reticulate,  often  with  numerous  short  hairs 
extending  out  from  surface  cells,  entirely  pseudoparenchymatous, 
outer  layer  of  darker  thick-walled  cells,  89.6  to  313.6  /*. 

Asci  clavate,  basal,  evanescent,  8-spored,  approximately  72  to 
96  /x  by  19.2  to  24  p..  Paraphyses  present  but  evanescent. 

Ascospores  muriform,  dark,  obtuse  at  each  end,  usually  with  4  to  7 
transverse  and  1  to  2  longitudinal  septa,  24  to  36  ju,  by  9.6  to  14.4  /*. 

Its  conidial  form  is  the  following: 

Volutella  circinans  comb.  nov. 
Vermicularia  circinans  Berk. 

Sporodochia  scattered  or  often  in  concentric  circles  usually  in 
the  centers  of  infected  areas,  numerous,  black,  subepidermal,  erum- 
pent,  becoming  covered  with  loose  mycelium,  1  to  2  mm.  in  diameter, 
1  mm.  in  elevation. 

Mycelium  hyaline,  becoming  dark,  rather  coarse,  3.6  to  10.8  ^ 
wide,  branching  irregularly,  and  with  characteristic  darkening  of  end 
cells  where  mycelium  is  superficial. 

Setae  one  to  many,  scattered  thruout,  dark  brown  to  black,  125 
to  240  /i  long,  4  /x  wide  at  base  tapering  to  apex. 

Conidiophores  straight,  simple,  hyaline,  few-septate,  obtuse,  24 
to  48  by  2.4  /*,  bearing  conidia  acrogenously. 

Conidia  falcate,  acute  at  each  end,  continuous,  hyaline,  19.2 
to  26.4  fji  by  3.6  to  7.2  /*. 


1919}  BLACK  SPOT  OP  ONION  SETS  531 

SUGGESTIONS   REGARDING   CONTROL 

This  Volutella  reaches  its  maximum  conidial  development  when 
it  grows  for  a  considerable  time  on  succulent  onions,  as  for  ex- 
ample on  an  inner  scale,  or  rarely  on  an  outer  scale  if  conditions 
are  such  that  this  remains  succulent.  When  drying  of  the  host 
tissue  occurs  soon  after  infection,  normal  conidial  development 
is  precluded.  Thus  it  happens  that  infection  of  the  outer  scale, 
which  in  a  comparatively  short  time  dries  to  papery  thinness,  leads 
in  most  cases  merely  to  the  production  of  mycelium  forming  dark 
blotches,  of  subicles  bearing  few  to  many  setae,  of  sterile  tubercles, 
and  of  sporodochia;  but  rarely  is  there  sufficient  time  and  succu- 
lence for  the  production  of  conidiiferous  sporodochia  in  any  abun- 
dance. These  conditions  obtain  in  what  has  been  here  designated 
as  Types  A  and  B,  and  account  for  the  difference  between  these 
types  and  Type  C.  Infection  can  pass  readily  from  an  outer  scale 
to  an  inner  scale  and  Type  A  can  thus  give  rise  to  Type  C,  tho 
Type  B  does  not  often  do  so. 

The  presence  of  atmospheric  moisture,  including  in  that  term 
the  moisture  in  the  air  between  the  scales,  retards  drying  of  the  in- 
fected scale,  thereby  lengthening  the  growth  period  of  the  fungus 
and  increasing  the  damage  done.  That  the  progress  of  the  fungus 
within  a  still  succulent  scale  may  be  stopped  by  atmospheric  drying 
is  shown  in  some  examples  of  Type  B  (see  Fig.  5).  Here  the  ad- 
vance of  the  fungus  has  been  checked,  as  is  indicated  by  the  line 
of  demarcation  between  dried  and  succulent  tissue,  by  simply  plac- 
ing the  onion  in  dry  air. 

From  these  observations  it  is  obvious  that  moisture  favors  the 
rot,  which  is  indeed  a  common  observation  of  practical  growers.  It 
allows  the  fungus  to  develop  longer  in  the  outer  scale,  perhaps  to 
make  some  spores  there,  and  surely  to  infect  the  lower  scales,  mak- 
ing many  spores  and  developing  a  genuine  rot.  Dry  air,  on  the 
other  hand,  hastens  the  drying  and  maturing  of  the  outer  scale,  per- 
haps entirely  prevents  spore  formation,  and  imprisons  the  fungus, 
even  preventing  it  from  proceeding  to  the  inner  scales. 

It  is  therefore  advisable  to  dry  onion  sets  as  rapidly  and  thoroly 
as  possible  immediately  after  they  are  harvested.  The  usual  method 
of  curing  in  shallow  slatted  crates  stacked  in  the  field  has  been 
found  inadequate  when  the  weather  is  moist.  Artificial  drying  of 
the  sets  in  rooms  properly  equipped  for  the  purpose  appears  to  be 
the  most  promising  method  for  checking  the  progress  of  this  dis- 
ease and  reducing  the  loss  from  rot.  The  desirability  of  properly 
curing  the  onions  to  prevent  loss  from  this  disease  was  mentioned 


532  BULLETIN  No.  220 

by  Thaxter  in  1889,  by  Halsted  in  1890,  by  Clinton  in  1903,  by  Stevens 
and  Hall  in  1910,  and  by  Walker  (see  footnote  p.  529).  Thus,  the 
recommendation  that  the  sets  be  dried  rapidly  and  thoroly  is  not  a 
new  one ;  nevertheless  it  is  worthy  of  careful  consideration  by  growers 
who  have  been  meeting  with  losses  on  account  of  the  disease  in 
question. 

BIBLIOGRAPHY 

1.  BERKELEY.     Gard.  Chron.,  595.     1851. 

2.  CLINTON,  G.  P.    Conn.  Agr.  Exp.  Sta.  Rpt.  27,  333,  Plate  20,  Fig.  a.     1903. 

3.  COOKE,  M.  C.     Handbook  of  British  Fungi,  439.     1871. 

4.  HALSTED,  B.  D.     N.  J.  Agr.  Exp.  Sta.  Rpt.  11,  354.     1890. 

5.  MASSEE,  G.  E.     Diseases  of  Cultivated  Plants,  417,  Fig.  130.     1910. 

6.  SELBY,  A.  D.     Ohio  Agr.  Exp.  Sta.  Bui.  214,  413,  Fig.  64.     1910. 

7.  STEVENS,  F.  L.     Fungi  Which  Cause  Plant  Disease,  497.     1913. 

8.  STEVENS  AND  HALL.     Diseases  of  Economic  Plants,  252.     1910. 

9.  STONEMAN,  BERTHA.     Bot.  Gaz.,  26,  69.     1898. 

10.  THAXTER,  R.    Conn.  Agr.  Exp.  Sta.  Rpt.  13,  163.     1889. 

11.  TUBEUF  AND  SMITH.     Diseases  of  Plants,  470.     1897. 

12.  VOGLINO,  P.     Annali  della  R.  Academia  d 'Agricolture  di  Torino,  49,  197. 

1907. 

13.  ZUKAL,  H.      Mykologische  Mittheilungen.      Oesterr.  Bot.   Zeit.,  43,   160-66. 

1893.     Also  in  Engler  and  Prantl.  Die  Naturlichen  Pflanzenfamilien,  1, 
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